Wind turbines are usually equipped with lightning protection systems to ensure that the large currents induced by a strike of lightning can be conducted to ground without damaging the components of the wind turbine.
The parts exposed the most to lightning are the blades. It is therefore important, that the lightning protection system provides a conduction path from the blades to the ground.
Wind turbines with a gear reduction placed between the rotating blade hub and the generator typically have large main bearings which gives a robust situation for a lightning protection system. Usually a current is led from the blades through the bearings, the shaft and some kind of spark gap and further to the ground.
A direct drive wind turbine without reduction gear and large bearings does not offer favourable conditions for a lightning protection system especially if an outer rotor is utilized.
EP 1 930 586 A1 shows a lightning protection system for wind generators with a conduction path from a wire conductor inside a blade to a metal strip at a root end of the blade and further to a gutter attached to the housing of the nacelle. A lightning transmission element is fixed to the hub thereby defining two spark gaps. One spark gap is arranged between the metal strip and the lightning transmission element while the second spark gap is arranged between the lightning transmission element and the gutter.